I'll have to check the code at home, but I would think something could me made relatively easy. It would be initially tailored specifically for your need. Then variables exposed so you were able to tweak it without the need to rebuild or need a coder. Bare with me I'm horrid with the proper terminology. I know in Multibox with Windows OS what happened was the screen is set to a "virutal?" screen size. Meaning Windows mapped the screen to their own coordinates. This to me was ignorant. I'm used to using screen resolution for everything, but this uses 65535x65535 regardless of resolution on the monitor.

So intial thoughts, you need a small program that has a mousehook. Also stores the current mouse position, and "the other" mouse position. Now upon clicking the mouse button, the program would send the mouse click to the screen, then would change the mouse potion to "the other" position, send another click. Then change mouse position back. I'm sure you could draw both at the same time, but why bother with the overhead.

I don't think I'm overly simplifing it, but I could be. I beleive you'll have 2 WoW running one on display. So 1600x1200 display.... You'll have two instance of WoW running at 1600x600? If so then you can Just take current mouse position - 65535/2 = top mouse position. Could even safe guard it...
[code:1]
if(MPos.Y > 65535/2)
{
// We on the bottom WoW
OtherPos.Y = MPos.Y - 65535/2;
}
else
{
// We must be on the top WoW
OtherPos.Y = MPos.Y + 65535/2;
}
[/code:1]

Hmm, am I correct on my assumptions of your setup? Does this sound like what you want?